The present invention has been developed in connection with offshore platforms of the type which is permanently located on the sea bottom and is involved in the drilling for hydrocarbons in the sea bed, or for the treatment of such products. The platforms are constructed for installation at a sea depth larger than 300 meters, but this should not be a limitation of the present invention.
Further, the present invention is developed in connection with so-called compliant offshore structures attached permanently to the bottom. Compliant platform structures must be designed in such a manner that they obtain a first fundamental period, which is so much longer (30-50 seconds) than the maximum wave period to which they are subjected, that the dynamic stress influence will become less than the corresponding static condition. Therefore, the platform design must be so "soft" that the period of movement of the platform will not to any degree become the same as the period of the influencing wave, which means that the platform will be compliant to the wave. When being subjected to stresses from ocean currents, waves and wind, the platform might tilt approximately 2-3 degrees at a water depth of 300 meters during a 100-years storm, whereas the corresponding tilt degrees are less at larger depths.
There are previously known platform structures in which the compliant effect is obtained by means of buoyancy tanks, buoyancy tanks combined with guylines and buoyancy tanks combined with various pile solutions etc.
External guylines suffer from the disadvantage of preventing free traffic around the platform and make it difficult for larger crane vessels to dock at and leave the platform.
In connection with compliant piles or telescopic piles one is faced with the problem that even if there is achieved unobstructed traffic around the platform structure, the compliant pile solution will entail a large quantity of piles with associated increased costs and a prolonged building period. The prolonged building period is mainly due to the fact that the piles must be installed after the completion of the trusswork structure.
Compliant platforms based on buoyancy tanks alone will result in unfavourably large structures, which in turn influence both costs and building period.